I feel like my DMing skills are making the game less enjoyable

I’m DMing a party of five, a rogue, cleric, wizard, fighter, and a ranger that comes every other week. Everything goes well for the first hour and a half, but then after that I can tell my narrations and stuff gets really bad. It’s the difference between ‘you shoot your bow, but your arrow had warped after that last river and you barely miss the knight’ and ‘you miss’. Since we play for about three hours a session and don’t come back together until the next week, the campaign always seems to end in a very bland spot, and sometimes the players have severely misinterpreted the surroundings(honestly, I don’t blame them when my descriptions turn into ‘you exit the building’). Another thing I can tell is that after while DMing, I get tired and my responses to the players get slower and slower and I have to consult the books more often. If it’s been a long day, sometimes I take fifteen minutes getting back to where we were and starting up again, and fall into the bland descriptions pretty quickly.

The players say they don’t mind much, but I feel bad for them since there is another group a table away whenever we’re playing, and the DM there is better than I am. I do give them a decent amount of loot and the occasional magic item and everything is balanced in the campaign. Still, I want to try and change my bad DMing so the campaign is less…wavy(if that makes any sense) in terms of detail. I’ve played with some rather bad DMs before, and know that it’s not fun when there’s little or no color to the adventure. I keep the action going though, plenty of mysteries and combat encounters, but again, it gets bland quickly.

Summary- Campaign is fine(mechanically), but I as DM get boring by the middle of the session and then everything slows down.

So I guess what I’m looking for a way to try and keep myself from getting boring by the end of the session and a way to keep the campaign going smoothly. Any suggestions?